Large-aperture infrared metalens camera

ABSTRACT

The disclosure discloses a large-aperture infrared metalens camera, which belongs to the technical field of infrared imaging and micro-nano photonics, including a large-aperture metalens, an infrared focal plane array detector, a metalens mechanical assembly and a housing. The large-aperture metalens has an aperture greater than 50 mm and a thickness less than 2 mm, and the distance between the large-aperture metalens and the infrared focal plane array detector is greater than 30 mm. The metalens mechanical assembly uses a buffer structure to fix, adjust and protect the metalens from shocks. The housing is sealed through a thermal insulation coating, thus providing heat insulation and waterproof protection for the lens. The disclosure adopts strict electromagnetic field values, diffraction design algorithm and large-area semiconductor process manufacturing method to increase the aperture of metalens to 50 mm or more, and considerably improves the focal length and magnification of the camera while ensuring that the F-number of the metalens meets the requirements of signal-to-noise ratio of image. The problems of short focal length, small magnification, and insufficient imaging range of conventional metalens cameras are overcome, thus realizing detection and imaging of objects at medium and long ranges.

BACKGROUND Technical Field

The disclosure belongs to the technical field of infrared imaging andmicro-nano photonics, and more specifically relates to a large-apertureinfrared metalens camera.

Description of Related Art

Infrared imaging technology is a technology that obtains the thermalradiation information of a target object and converts the thermalradiation information into an image visible to the human eye. Comparedwith visible light imaging technology, this imaging technology has theadvantages of high concealment, anti-interference ability, and goodadaptability to surroundings. Therefore, the technology is commonly usedin military fields such as night reconnaissance, infrared guidance, andearly warning of missile, as well as other civilian fields such assecurity monitoring, vehicle-mounted night vision, and industrialinspection.

In recent years, as infrared imaging technology is increasingly adoptedin airborne, mobile and other occasions that are sensitive to equipmentweight, volume and cost, light, compact and low-cost infrared camerashave become a focus of research and development. On the other hand,infrared cameras are also developed to have a larger focal length andmagnification in a limited space, so as to obtain a significantly largeroperating range and is expected to identify target objects at a fartherrange and further exploit the advantages of infrared imaging technology.In view of the above, to be lighter and to see farther are two majordevelopment trends of infrared imaging technology at present.

However, conventional lenses rely on the curved surface of componentsand the optical properties of materials to perform wavefront control oflight, making it difficult to further reduce the weight, volume and costof infrared cameras. As a new research direction in the field ofnanophotonics, electromagnetic metasurfaces are expected to replaceconventional lenses so that it is possible for infrared cameras to be“lighter”. In terms of structure, an electromagnetic metasurface is atwo-dimensional array of sub-wavelength or wavelength-scaleelectromagnetic resonance units; in terms of function, anelectromagnetic metasurface is able to adjust the intensity, frequency,phase, polarization and other parameters of electromagnetic waves in theentire electromagnetic spectrum. Electromagnetic metasurface-basedimaging technology (metalens) as a branch, compared with conventionaloptical components, has a lighter structure, lower cost, and is moreadaptable for plane processing technology, so such imaging technologyhas broad prospects in application. If the metalens is used for infraredimaging, it helps to reduce the weight, volume and cost of the infraredcamera.

Unfortunately, the current infrared camera with metalens is not able toachieve the two goals of “being lighter” and “seeing further”simultaneously for the following reasons:

(1) In the current design of infrared camera with metalens, it is almostimpossible to design an infrared metalens with an aperture greater than50 mm. At present, strict electromagnetic field numerical algorithms(such as finite time domain difference algorithm) are generally adoptedto simulate the focusing spot of infrared metalens, which is used as thebasis for the design of infrared metalens. However, when the aperture ofthe infrared metalens increases to 50 mm, millions of columnarstructural units are required in the design, and such a large-scalesimulation model is almost impossible to run on a general engineeringdesign computer with an acceptable time cost.

(2) In the current manufacturing process of infrared cameras withmetalens, it is a big problem to manufacture infrared metalens with anaperture larger than 50 mm. At present, electron beam exposure orultraviolet projection lithography is commonly used in the patterningprocess to manufacture infrared metalenses. However, electron beamlithography is a point-by-point exposure process, which cannot completethe patterning of millions of columnar structural units with acceptabledimensional accuracy and time cost. Although ultraviolet projectionlithography can guarantee a high yield, restricted by the field of viewof a projection lens, the area of one exposure of UV projectionlithography is limited (usually 20 mm*20 mm) and unable to cover theexposure area required by large-aperture metalens (at least 50 mm*50mm).

(3) In combination of the above two problems, the aperture of thecurrent infrared camera with metalens is limited. With the requirementthat the F-number of metalens has to satisfy the image signal-to-noiseratio, the focal length and magnification of metalens will also belimited. As a result, the imaging range (or operating range) is alsolimited, which means that the current infrared camera with metalens isunable to achieve the goal of “being lighter” and “seeing further”, andthere is a conflict between weight and imaging range (or operatingrange).

SUMMARY

In view of the above defects or needs of improvement in the related art,the present disclosure provides a large-aperture infrared metalenscamera, thereby solving the technical problems of small focal length,low magnification, and insufficient imaging range of the currentinfrared camera with metalens.

To achieve the above purpose, in an aspect of the present disclosure, alarge-aperture infrared metalens camera is provided, including alarge-aperture metalens, an infrared focal plane array detector, ametalens mechanical assembly and a housing.

The large-aperture metalens is arranged on the metalens mechanicalassembly, and the metalens mechanical assembly is assembled on thehousing. The housing is provided with a telescopic member that can moveaxially along the mirror surface of the large-aperture metalens.

The metalens mechanical assembly is configured to fix the large-aperturemetalens.

The telescopic member is configured to move the large-aperture metalensaxially along its mirror surface, so that the distance between thelarge-aperture metalens and the infrared focal plane array detector isgreater than 30 mm.

The large-aperture metalens is configured to bend the light of thermalradiation of the target object and converge the light onto the surfaceof the infrared focal plane array detector, and large-aperture metalenshas an aperture greater than 50 mm and a thickness less than 2 mm.

The infrared focal plane array detector is configured to eliminate straylight and light outside the wavelength band of detection, so as torealize detection and imaging.

Preferably, the large-aperture metalens includes a metasurfacemicrostructure array, a microstructure array film coating, a substrateand a substrate film coating.

The microstructure array film coating is coated on the surface of themetasurface microstructure array to increase transmittance for anincident light, and the surface shape thereof is the same as that of themetasurface microstructure array.

The substrate film coating is coated on the surface of the substrate toincrease transmittance for an incident light.

The metasurface microstructure array is located on the rear surface ofthe substrate, and the rear surface of the substrate is a surface wherethe light arrives later along the incident-light direction.

Preferably, the metasurface microstructure array includes a plurality ofcolumnar structural units arranged according to an ordered latticeperiodicity, where the heights of the columnar structural units are allthe same, and fall within the order of the detected wavelength. Thediameters of the columnar structural units fall within the order ofsubwavelength.

The periodicity of the array composed of the columnar structural unitsis less than 10 microns, and the diameter of the array composed of thecolumnar structural units is equal to the aperture of the large-aperturemetalens.

Preferably, the material of the columnar structural units includessilicon or germanium; the material of the microstructure array filmcoating includes zinc sulfide or germanium; the material of thesubstrate includes intrinsic double-thrown silicon, zinc sulfide orbarium fluoride; and the material of the substrate film coating includeszinc sulfide or germanium.

Preferably, the aperture and F-number of the large-aperture metalens aredetermined according to the following formulae:

$C = \frac{1}{( {\frac{L}{D \times F}\text{- 1}} ) \times P} > C_{d}$

$SNR = K \times \frac{P}{4F^{2}} > SNR_{d}$

In the formulae, C is a pixel density of the target object in adetection image, L is a distance between the target object and thelarge-aperture infrared metalens camera, D is an aperture of thelarge-aperture metalens, F is the F-number of the large-aperturemetalens, P is a pixel spacing of the infrared focal plane arraydetector, C_(d) is the required minimum pixel density of the targetobject in the detection image; SNR is the signal-to-noise ratio ofdetection of camera, K is a parameter related to the radiation degree ofthe target object, detection surroundings, lens transmittance, anddetector responsivity, and SNR_(d) is the required minimumsignal-to-noise ratio of the camera.

Preferably, the metasurface microstructure array (101) is designed bythe following method: optimally designing and obtaining the surfacephase distribution of the large-aperture metalens by using a diffractiondesign algorithm or a ray tracing algorithm according to the apertureand F-number of the large-aperture metalens; obtaining the relationshipbetween the phase and transmittance of the columnar structural units andthe size of the columnar structural units; determining the size of eachof the columnar structural units at each position in the metasurfacemicrostructure array according to the surface phase distribution of thelarge-aperture metalens and the relationship between the phase andtransmittance of the columnar structural units and the size of thecolumnar structural units, and using the diffraction design algorithm orthe ray tracing algorithm again for feedback optimization; the infraredtransmittance of the columnar structural units is greater than therequired value of the infrared transmittance.

In a preferred embodiment, the microstructure array film coating isoptimally designed through an electromagnetic field simulation algorithmaccording to the shape of the metasurface microstructure array.

In a preferred embodiment, the substrate is optimally designed by usinga finite difference time domain method and a ray tracing method.

Preferably, the metasurface microstructure array is manufactured byusing semiconductor technology, including but not limited to steppingphotolithography, step-scanning photolithography, nanoimprinting, laserdirect writing, metal lift-off or ICP etching.

The microstructure array film coating and the substrate film coating aremanufactured by an optical coating process, including but not limited toelectron beam evaporation coating.

The substrate is processed through optical polishing.

Preferably, the infrared focal plane array detector includes a detectorwindow and an infrared focal plane array, and the detector window andthe infrared focal plane array are sequentially arranged along theincident-light direction.

The detector window is configured to filter out stray light and lightoutside the wavelength band of detection.

The infrared focal plane array is configured to detect and image theconverged light.

Preferably, the metalens mechanical assembly includes a bufferstructure, the buffer structure is provided with a groove matching theedge of the large-aperture metalens, and the groove is provided with amechanical damping member configured to fix and protect thelarge-aperture metalens against shocks.

The material of the mechanical damping member includes but not limitedto rubber, composite material or high damping alloy.

Preferably, the contact surfaces where the housing is in contact withthe metalens mechanical assembly and the infrared focal plane arraydetector are respectively provided with sealing gaskets.

The housing is provided with a thermal insulation coating, and thematerial of the thermal insulation coating includes but not limited tometal oxide micro-powder or non-metallic hollow micro-spheres.

Generally speaking, compared with the related art, the above technicalsolutions conceived by the present disclosure are able to achieve thefollowing advantageous effects:

1. In the large-aperture infrared metalens camera provided by thepresent disclosure, the aperture of the large-aperture metalens isgreater than 50 mm, and the focal length is greater than 30 mm. On thepremise of keeping the large-aperture metalens light, the magnificationand imaging range are considerably improved, so the problems of shortfocal length and low magnification of conventional metalens cameras areovercome, and it is possible to detect and image objects at medium andlong ranges. The disclosure further obtains the method of determiningthe aperture and F-number of the large-aperture metalens according tothe system parameters, thereby better taking into account the goals of“being lighter” and “seeing farther”, and solving the conflict betweenweight and imaging range (or operating range).

2. The technology for designing the large-aperture infrared metalenscamera provided by the present disclosure does not need to complicatedlycalculate the strict electromagnetic field of the entire large-aperturemetalens. Instead, the technology divides the design of thelarge-aperture metalens into surface phase distribution design and localphase design (i.e., phase design of columnar structural units), and itis only necessary to calculate the strict electromagnetic field of thecolumnar structural units and the diffraction field or light field ofthe surface phase distribution, which considerably reduces thecalculation steps and improves the design efficiency. In terms of thesimulation feedback optimization, the diffraction algorithm or the raytracing algorithm is also introduced to replace the strictelectromagnetic field algorithm, which further improves the calculationaccuracy and improves the optimization efficiency.

3. The large-aperture infrared metalens camera provided by the presentdisclosure adopts high-yield large-area patterning processes such asstepping lithography, step-by-step scanning lithography, nanoimprintingor laser direct writing to replace small-area pattering processes suchas electron beam exposure and ultraviolet projection lithographyprocess, thereby expanding the coverage area of the patterning process,increasing the speed of patterning, so that the large-aperture metalensmay be fabricated in large quantities.

4. In the large-aperture infrared metalens camera provided by thepresent disclosure, the large-aperture metalens is coated with ananti-reflection film on both sides, which improves the transmittance ofthe metalens. The mechanical assembly of the metalens adopts a bufferstructure, which is able to fix, adjust and protect the metalens againstshocks, thus solving the problem of poor mechanical performance ofconventional metalens cameras.

5. The housing of the large-aperture infrared metalens camera providedby the present disclosure adopts a thermal insulation coating andsealing treatment, thereby protecting the lens from heat and water, sothat the lens has better athermalization and waterproof performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic structural view of a large-aperture infraredmetalens camera provided by an embodiment of the present disclosure.

FIG. 2 is a schematic structural view of a large-aperture metalensprovided by an embodiment of the present disclosure.

FIG. 3 is a schematic view of a columnar structural unit and a substratein the form of a hexagonal lattice provided by an embodiment of thepresent disclosure.

FIG. 4 is a top view of a columnar structural unit and a substrate inthe form of a hexagonal lattice provided by an embodiment of the presentdisclosure.

FIG. 5 is a surface phase distribution of a large-aperture metalensprovided by an embodiment of the present disclosure.

FIG. 6 shows the phase and transmittance obtained under differentdiameters of the columnar structural units provided by an embodiment ofthe present disclosure.

FIG. 7 is a top view of a large-area metasurface microstructure array inthe large-aperture metalens designed and provided by an embodiment ofthe present disclosure.

FIG. 8 is a partial top view of a large-area metasurface microstructurearray manufactured by a process provided by an embodiment of the presentdisclosure.

FIG. 9 is a partial oblique view of a large-area metasurfacemicrostructure array manufactured by a process provided by an embodimentof the present disclosure.

FIG. 10 is a schematic view of four large-aperture metalenses 1 on a6-inch silicon wafer manufactured by a process provided by an embodimentof the present disclosure.

FIG. 11 is a schematic structural view of a metalens mechanical assemblyprovided by an embodiment of the present disclosure.

FIG. 12 is a schematic structural view of a housing provided by anembodiment of the present disclosure.

FIG. 13 is a sample photo of the large-aperture metalens provided by anembodiment of the present disclosure.

FIG. 14 is a schematic view showing the comparison between the indoor 50m long-range imaging experiment result and the recognition effect of thevisible light short-focus lens provided by an embodiment of the presentdisclosure.

In all the drawings, the same reference numerals are used to representthe same members or structures, in which: 1-large-aperture metalens;101-metasurface microstructure array; 1011-columnar structural unit;102-microstructure array film coating layer; 103-substrate;104-substrate film coating; 2-infrared focal plane array detector;3-metalens mechanical assembly; 301-buffer structure; 3011-mechanicaldamping member; 3012-groove; 4-housing; 401-sealing gasket; 5-telescopicmember; 6-thermal insulation coating.

DESCRIPTION OF THE EMBODIMENTS

In order to make the purpose, technical solution and advantages of thepresent disclosure more comprehensible, the present disclosure will befurther described in detail below in conjunction with the accompanyingdrawings and embodiments. It should be understood that the specificembodiments described here are only used to explain the presentdisclosure, not to limit the present disclosure. In addition, thetechnical features involved in the various embodiments of the presentdisclosure described below can be combined with each other as long asthey do not constitute a conflict with each other.

As shown in FIG. 1 to FIG. 14 , the present disclosure provides alarge-aperture infrared metalens camera, including a large-aperturemetalens 1 and an infrared focal plane array detector 2 arranged insequence along the incident-light direction, as well as a metalensmechanical assembly 3 and a housing 4.

The large-aperture metalens 1 is configured to bend the light of thethermal radiation of the target object, and has a diameter greater than50 mm and a thickness less than 2 mm. By using the large-aperturemetalens 1 with a small thickness for imaging, on the premise of keepingthe large-aperture metalens 1 light, the magnification and imaging rangeare considerably improved, so the problems of short focal length and lowmagnification of conventional metalens cameras are overcome, and it ispossible to detect and image objects at medium and long ranges.Preferably, the aperture and F-number of the large-aperture metalens 1are determined according to the following formulae:

$C = \frac{1}{( {\frac{L}{D \times F}\text{-1}} ) \times P} > C_{d}$

$SNR = K \times \frac{P}{4F^{2}} > SNR_{d}$

In the formulae, C is a pixel density of the target object in adetection image, L is a distance between the target object and thelarge-aperture infrared metalens camera, D is an aperture of thelarge-aperture metalens, F is the F-number of the large-aperturemetalens, P is a pixel spacing of the infrared focal plane arraydetector, C_(d) is the required minimum pixel density of the targetobject in the detection image; SNR is the signal-to-noise ratio ofdetection of camera, K is a parameter related to the radiation degree ofthe target object, detection surroundings, lens transmittance, anddetector responsivity, and SNR_(d) is the required minimumsignal-to-noise ratio of the camera. The above determination criteriatake into account both the weight parameter of the camera (the apertureand F-number of the large-aperture metalens) and the imaging rangeparameter (the distance between the target object and the large-apertureinfrared metalens camera), so as to better take into account the goalsof “being lighter” and “seeing further”, thus solving the conflictbetween weight and imaging range (or operating range).

In an embodiment of the present disclosure, the large-aperture metalens1 includes a metasurface microstructure array 101, a microstructurearray film coating 102 covering the metasurface microstructure array101, a substrate 103, and a substrate film coating 104 covering thesubstrate 103. The metasurface microstructure array 101 is located onthe rear surface of the above-mentioned substrate 103, and the rearsurface of the above-mentioned substrate 103 is a surface where thelight arrives later along the incident-light direction. The metasurfacemicrostructure array 101 is composed of a series of columnar structuralunits 101 arranged according to an ordered lattice periodicity, wherethe heights of the columnar structural units 1011 are all the same, andfall within the order of the detected wavelength. The diameters of thecolumnar structural units 1011 fall within the order of subwavelength.The periodicity of the array composed of the columnar structural units1011 is less than 10 microns, and the diameter of the array composed ofthe columnar structural units 1011 is equivalent to the aperture of thelarge-aperture metalens 1. The material of the columnar structural units1011 is an infrared high-refractive-index material, including but notlimited to silicon, germanium, etc. The microstructure array filmcoating 102 covering the metasurface microstructure array is configuredto increase transmittance for an incident light, and a surface shapethereof changes with the shape of the metasurface microstructure array101, including but not limited to zinc sulfide, germanium coating. Thesubstrate 103 is made of a material highly transparent to infraredlight, including but not limited to intrinsic double-polished silicon,zinc sulfide, barium fluoride, etc. The substrate film coating 104covering the substrate is provided to increase transmittance for anincident light, including but not limited to zinc sulfide, germaniumcoating.

In an embodiment of the present disclosure, the large-aperture metalens1 is coated with an anti-reflection coating on both sides, whichincreases the transmittance of the camera.

To further illustrate, the metasurface microstructure array 101 isdesigned as follows: optimally designing and obtaining the surface phasedistribution of the large-aperture metalens 1 by using a diffractiondesign algorithm or a ray tracing algorithm according to the apertureand F-number of the large-aperture metalens 1; obtaining therelationship between the phase and transmittance of the columnarstructural units 1011 and the size of the columnar structural units 1011according to the strict electromagnetic field numerical algorithm;determining the size of each of the columnar structural units 1011 ateach position in the metasurface microstructure array 101 according tothe surface phase distribution of the large-aperture metalens 1 and therelationship between the phase and transmittance of the columnarstructural units 1011 and the size of the columnar structural units1011, and using the diffraction design algorithm or the ray tracingalgorithm again for feedback optimization; the infrared transmittance ofthe columnar structural units 1011 is greater than the required value ofthe infrared transmittance; the microstructure array film coating 102covering the metasurface microstructure array is optimally designedaccording to the shape of the metasurface microstructure array 101through an electromagnetic field simulation algorithm; the substrate 103is optimally designed by using the finite difference time domain methodand ray tracing method. The above design method does not need tocomplicatedly calculate the strict electromagnetic field of the entirelarge-aperture metalens. Instead, the design divides the large-aperturemetalens into surface phase distribution design and local phase design(i.e., phase design of columnar structural units), and it is onlynecessary to calculate the strict electromagnetic field of the columnarstructural units and the diffraction field or light field of the surfacephase distribution, which considerably reduces the calculation steps andimproves the design efficiency. In terms of the simulation feedbackoptimization, the diffraction algorithm or the ray tracing algorithm isalso introduced to replace the strict electromagnetic field algorithm,which further improves the calculation accuracy and improves theoptimization efficiency.

In an embodiment of the present disclosure, the metasurfacemicrostructure array 101 is fabricated by a large-area semiconductorprocess, including but not limited to stepping photolithography,step-by-step scanning photolithography, nanoimprinting, laser directwriting, metal lift-off, ICP etching, etc. The microstructure array filmcoating 102 covering the metasurface microstructure array and thesubstrate film coating 104 covering the substrate 103 are fabricated byan optical coating process, including but not limited to electron beamevaporation coating. The substrate 103 is optically polished. In theabove fabrication process, high-yield large-area patterning processessuch as stepping lithography, step-by-step scanning lithography,nanoimprinting or laser direct writing are adopted to replace small-areapatterning processes such as electron beam exposure and ultravioletprojection lithography processes, thereby expanding the coverage area ofthe patterning process, increasing the speed of patterning, so thatlarge-aperture metalens may be fabricated in large quantities.

To further illustrate, the infrared focal plane array detector 2 isconfigured to eliminate stray light and light outside the wavelengthband of detection, so as to realize detection and imaging. In anembodiment of the present disclosure, the infrared focal plane arraydetector 1 includes a detector window and an infrared focal plane arrayarranged in sequence along the incident-light direction. The detectorwindow is configured to filter out stray light and light outside thewavelength band of detection. The infrared focal plane array isconfigured to detect and image the converged light.

In an embodiment of the present disclosure, the metalens mechanicalassembly 3 is configured to fix, adjust and protect the large-aperturemetalens 1 against shocks. Specifically, the metalens mechanicalassembly 3 is provided with a buffer structure 301, and the bufferstructure 301 adopts a mechanical damping member 3011. The material ofthe mechanical damping member 3011 includes but is not limited torubber, composite material, high damping alloy, etc. The bufferstructure 301 has a groove 3012 matching the edge of the large-aperturemetalens 1, and the groove 3012 is able to clamp the large-aperturemetalens 1 to fix and protect the large-aperture metalens 1 againstshocks. The metalens mechanical assembly 3 is fixedly connected with themetalens mechanical assembly 3 and the housing 4. The housing 4 may bestretched through the telescopic member 5, so that the metalensmechanical assembly 3 cooperates with the housing 4 to adjust thelarge-aperture metalens 1. The metalens mechanical assembly 3 isprovided with a buffer structure 301, which may fix, adjust and protectthe large-aperture metalens 1 against shocks, thus solving the problemof poor mechanical performance of conventional metalens cameras.

In an embodiment of the present disclosure, the housing 4 serves toprovide thermal insulation and waterproof protection for the lens.Preferably, the housing 4 is sealed with a thermal insulation coating 6.The material of the thermal insulation coating 6 includes, but is notlimited to, metal oxide micro-powder, non-metallic hollow micro-spheres,etc., and the sealing process includes, but is not limited to, the useof sealing gaskets 401 at the joints of the housing 4. The housing 4enables the lens to have better athermalization and waterproofperformance.

In the present disclosure, the distance between the large-aperturemetalens 1 and the infrared focal plane array detector 2 is greater than30 mm. When there is no other optical elements added between thelarge-aperture metalens 1 and the infrared focal plane array detector 2,the distance provides a necessary space for the focal length of thecamera, which is essential for increasing the magnification of thecamera.

The disclosure provides a large-aperture infrared metalens camera, whichimproves the design efficiency of large-aperture metalens in terms ofdesign technology. In terms of fabrication technology, it is possiblefor large-aperture metalens to be fabricated in large quantities, andtherefore the aperture of the large-aperture metalens may be greaterthan 50 mm, and the focal length may be greater than 30 mm. On thepremise of keeping the large-aperture metalens light, the magnificationand imaging range are considerably improved, thereby solving thetechnical problems of small focal length, low magnification andinsufficient imaging range of current infrared cameras with metalens. Inthe meantime, the disclosure retains the advantages of metalens beinglight and thin (thickness less than 2 mm) and being able to bemass-produced, thus allowing the camera to be light, compact and low incost, significantly reducing the weight, volume and cost of the mediumand long-range infrared camera. The disclosure may be applied to mediumand long-range detection and imaging, border security, medium andlong-range thermal induction, smart home, intelligent environmentperception and other occasions.

The technical solutions of the present disclosure will be furtherdescribed below through specific examples.

FIG. 1 illustrates a large-aperture infrared metalens camera provided bythe present disclosure, including: a large-aperture metalens 1, aninfrared focal plane array detector 2, a metalens mechanical assembly 3,a housing 4, a telescopic member 5, and a thermal insulation coating 6.

The large-aperture metalens 1 is configured to bend the light of thethermal radiation of the target object, and has a thickness of 0.5 mm.The aperture and F-number of the large-aperture metalens 1 are furtherselected according to the following formulae:

$C = \frac{1}{( {\frac{L}{D \times F}\text{- 1}} ) \times P} > C_{d}$

$SNR = K \times \frac{P}{4F^{2}} > SNR_{d}$

In the formulae, C is a pixel density of the target object in adetection image, L is a distance between the target object and thelarge-aperture infrared metalens camera, D is an aperture of thelarge-aperture metalens, F is the F-number of the large-aperturemetalens, P is a pixel spacing of the infrared focal plane arraydetector, C_(d) is the required minimum pixel density of the targetobject in the detection image; SNR is the signal-to-noise ratio ofdetection of camera, K is a parameter related to the radiation degree ofthe target object, detection surroundings, lens transmittance, anddetector responsivity, and SNR_(d) is the required minimumsignal-to-noise ratio of the camera.

In this embodiment, P=17 µm. According to the empirical formula of K andthe empirical value that is substituted, under the appropriate SNR_(d),F=1 is selected. According to the Johnson criterion of infrared images,it requires no less than 12 pixels to be imaged in the criticaldirection to identify the target. Assuming that this embodiment is usedfor medium and large indoor intelligent sensing (such as gymnasiums), itis necessary to detect the whole body thermal image of a person toidentify the human body and analyze the moving position and routethereof to provide information for controlling indoor equipment (such asair conditioners, lights, curtains, projectors, etc.) in the next step.The target object is a person, and the feature size exceeds 1 m.Considering that the camera also has certain aberrations and noise,C_(d)=12^(∗)4=48m⁻ ¹ is selected. When the imaging range L=50 m, F=1,P=17 µm, take D=50.2 mm, then C=59m⁻ ¹>C_(d). That is, the aperture ofthe large-aperture metalens is 50.2 mm, and the F-number is 1, which canmeet the requirements of the signal-to-noise ratio and the imaging range(not less than 50 meters) of this embodiment simultaneously. The abovedetermination criteria ensure that the appropriate parameters areselected under the required target parameters, while taking into accountthe weight parameters of the camera (the aperture and F-number of thelarge-aperture metalens) and the imaging range parameters (the distancebetween the target object and the above large-aperture infrared metalenscamera). In this way, it is possible to better take into account thegoals of “being lighter” and “seeing farther”, and solve the conflictbetween weight and imaging range (or operating range).

In further explanation, the specific structure of the large-aperturemetalens in the present embodiment is as shown in FIG. 2 , including themetasurface microstructure array 101, the microstructure array filmcoating 102 covering the metasurface microstructure array 101, thesubstrate 103 and the substrate film coating 104 covering the substrate103. The metasurface microstructure array 101 is located on the rearsurface of the substrate 103, and the rear surface of the substrate 103is a surface where the light arrives later along the incident-lightdirection. In this embodiment, in order to obtain a larger unit-to-phasesampling density, the metasurface microstructure array 101 is formed bya series of columnar structural units 1011 arranged in a hexagonallattice periodicity. FIG. 3 and FIG. 4 further respectively show theschematic view and top view of the columnar structural units 1011 and asubstrate 103 in the form of hexagonal lattice. It should be noted that,in order to better illustrate the structure of the columnar structuralunits, the drawings omit the microstructure array film coating 102covering the metasurface microstructure array 101 and the substrate filmcoating 104 covering the substrate 103. The heights of the columnarstructural units 1011 are all the same and fall within the order of thedetected wavelength, and the diameters of the columnar structural units1011 fall within the order of subwavelength. The periodicity of thearray formed by the columnar structural units 1011 is less than 10microns, and the diameter of the array composed of the columnarstructural units 1011 is equal to the aperture D of the large-aperturemetalens 1. In this embodiment, the operation wavelength band islong-wave infrared, so the material of the columnar structural units1011 is intrinsic silicon. The microstructure array film coating 102covering the metasurface microstructure array 101 is configured toincrease transmittance for an incident light, and the surface shapethereof is changed along with the shape of the metasurfacemicrostructure array 101, using alternating coatings of zinc sulfide andgermanium. The substrate 103 adopts an intrinsic double-polished siliconwafer. The substrate film coating 104 covering the substrate 103 is usedto increase transmittance for an incident light, and adopts alternatecoatings of zinc sulfide and germanium. The large-aperture metalens 1 iscoated with anti-reflection coating on both sides, which maytheoretically increase the transmittance of the camera to 80% or more.

The metasurface microstructure array 101 described in this embodiment isdesigned according to the following method: optimally designing andobtaining the surface phase distribution of the large-aperture metalens1 by using a ray tracing algorithm according to the aperture D=50.2 mmand F-number=1 of the large-aperture metalens 1; obtaining therelationship between the phase and transmittance of the columnarstructural units 1011 and the size of the columnar structural units 1011(which is the diameter of the cylinder in this example) according to atime domain finite difference algorithm (a commonly used strictelectromagnetic field numerical algorithm); determining the size of eachof the columnar structural units 1011 at each position in thelarge-surface metasurface microstructure array 101 according to thesurface phase distribution of the large-aperture metalens 1 and therelationship between the phase and transmittance of the columnarstructural units 1011 and the diameter of the columnar structural units1011, and using the diffraction design algorithm again for feedbackoptimization. Specifically, the phase and transmittance of the columnarstructural units 1011 are substituted into the diffraction designalgorithm, so that the simulation may reflect the optical performance ofthe metasurface microstructure array 101. The infrared transmittance ofthe columnar structural units 1011 is greater than the required value ofthe infrared transmittance. The microstructure array film coating 102covering the metasurface microstructure array 101 is optimally designedaccording to the shape of the metasurface microstructure array 101through a finite time domain difference algorithm; the thickness of thesubstrate 103 is optimally designed by using a ray tracing method.According to the above method, FIG. 5 shows the surface phasedistribution of the large-aperture metalens 1 obtained in the finaldesign. The height of the columnar structural units 1011 is selected as6 µm, and the periodicity is 4 µm. FIG. 6 shows the relationship betweenthe phase and transmittance of the columnar structural units 1011 andthe diameter of the columnar structural units 1011. FIG. 7 is a top viewof the metasurface microstructure array 101 in the designedlarge-aperture metalens 1. All simulation designs are completed on acommon computer, which verifies the high simulation design efficiency ofthe present disclosure for large-aperture metalens 1.

To further illustrate, the metasurface microstructure array 101 isfabricated by a large-area semiconductor process. In this embodiment,stepping photolithography and ICP etching are the two main processesthat are adopted. The stepping photolithography adopts 9 masks andcarries out 9 times of stepping photolithography, so that 9 exposureareas are spliced into an exposure area of the large-aperture metalens1, and the photoresist pattern of the metasurface microstructure array101 is generated. The ICP etching adopts the Bosch process, and thephotoresist pattern generated by stepping photolithography is used as anetching mask to obtain the metasurface microstructure array 101 with ahigh aspect ratio. FIG. 8 and FIG. 9 are partial top view and obliqueview of the metasurface microstructure array 101 respectivelymanufactured according to the above processes. The microstructure arrayfilm coating 102 covering the metasurface microstructure array 101 andthe substrate film coating 104 covering the substrate 103 are fabricatedby an optical coating process. In this embodiment, electron beamevaporation coating is adopted; the substrate is an intrinsicdouble-polished wafer, and both sides are polished. FIG. 10 shows fourlarge-aperture metalenses 1 fabricated on a 6-inch silicon wafer, whichverifies that the large-aperture metalens 1 of the present disclosure isable to be mass-produced on silicon wafers.

To further illustrate, the infrared focal plane array detector 2 isconfigured to eliminate stray light and light outside the wavelengthband of detection, so as to realize detection and imaging. The infraredfocal plane array detector 2 includes a detector window and an infraredfocal plane array arranged in sequence along the incident-lightdirection. The detector window is configured to filter out the straylight of the system and the light outside the wavelength band ofdetection. The infrared focal plane array is configured to detect andimage the converged light. The technical specifications of the infraredfocal plane array detector 2 adopted in this embodiment are as follows:the operation wavelength band is 8 to 14 µm; the pixel size is 17 µm;the array resolution is 1280×960.

To further illustrate, the metalens mechanical assembly 3 is configuredto fix, adjust and protect the large-aperture metalens 1 against shocks.As shown in FIG. 11 , the metalens mechanical assembly 3 includes abuffer structure 301, and the buffer structure 301 is provided with amechanical damping member 3011. In this embodiment, the mechanicaldamping member 3011 is made of rubber; the buffer structure 301 has agroove 3012 matching the edge of the metalens, and the groove 3012 isable to clamp the large-aperture metalens 1 and fix and protect thelarge-aperture metalens 1 against shocks. As shown in FIG. 1 , themetalens mechanical assembly 3 is fixedly connected with the housing 4,and the housing 4 is able to be stretched through the telescopic member5, so that the metalens mechanical assembly 3 and the housing 4 maycooperate to adjust the metalens.

To further illustrate, the housing 4 serves to provide thermalinsulation and waterproof protection for the lens. The housing 4 issealed with a thermal insulation coating 6. In this embodiment, thematerial of the thermal insulation coating 6 adopts a compositeemulsion, and the filler is ultrafine hollow micro-spheres, metal oxidemicro-powder and titanium dioxide. As shown in FIG. 12 , the sealingprocess in the embodiment is carried by using sealing gaskets 401 at thejoints of the housing 4.

To further illustrate, the distance between the large-aperture metalens1 and the infrared focal plane array detector 2 is greater than 30 mm.This distance is 50 mm in this embodiment.

In order to verify the weight and imaging range performance of thelarge-aperture infrared metalens camera of the present disclosure, theembodiment of the present disclosure is subjected to experimental tests.FIG. 13 is a sample photo of the large-aperture metalens 1 of thepresent embodiment, which verifies that the metalens aperture of thepresent disclosure is greater than 50 mm. The weight of thelarge-aperture metalens of this embodiment measured by an electronicweighing instrument is only 3.7 grams. The results of the indoor 50 mlong-range imaging experiment show that the long-range recognition ofthe target object (indicated by arrow) at 50 m away is feasible, asshown in FIG. 14 . Compared with the recognition effect of visible lightshort-focus lens, the infrared camera composed of metalens has a moreobvious recognition effect. According to this result, the embodiment ofthe present disclosure better balances the goals of “being lighter” and“seeing farther”, and solves the conflict between weight and imagingrange (or operating range).

It is easy for those skilled in the art to understand that the abovedescriptions are only preferred embodiments of the present disclosure,and are not intended to limit the present disclosure. Any modifications,equivalent replacements and improvements made within the spirit andprinciples of the present disclosure should be included within theprotection scope of the present disclosure.

What is claimed is:
 1. A large-aperture infrared metalens camera,comprising: a large-aperture metalens, an infrared focal plane arraydetector, a metalens mechanical assembly and a housing; wherein thelarge-aperture metalens is disposed on the metalens mechanical assembly,and the metalens mechanical assembly is assembled on the housing, thehousing is provided with a telescopic member that is movable axiallyalong a mirror surface of the large-aperture metalens; the metalensmechanical assembly is configured to fix the large-aperture metalens;the telescopic member is configured to move the large-aperture metalensaxially along the mirror surface thereof, so that a distance between thelarge-aperture metalens and the infrared focal plane array detector isgreater than 30 mm; the large-aperture metalens is configured to bend alight of thermal radiation of a target object and converge the lightonto a surface of the infrared focal plane array detector, and thelarge-aperture metalens has an aperture greater than 50 mm and athickness less than 2 mm; the infrared focal plane array detector isconfigured to eliminate stray light and light outside a wavelength bandof detection, so as to realize detection and imaging.
 2. Thelarge-aperture infrared metalens camera according to claim 1, whereinthe large-aperture metalens comprises a metasurface microstructurearray, a microstructure array film coating, a substrate and a substratefilm coating; the microstructure array film coating is coated on asurface of the metasurface microstructure array, and a surface shapethereof is the same as a shape of the metasurface microstructure array,which is configured to increase transmittance for an incident light; thesubstrate film coating is coated on a surface of the substrate so as toincrease transmittance for the incident light; the metasurfacemicrostructure array is located on a rear surface of the substrate,wherein the rear surface of the substrate is a surface where a lightarrives later along an incident-light direction.
 3. The large-apertureinfrared metalens camera according to claim 2, wherein the metasurfacemicrostructure array comprises a plurality of columnar structural unitsarranged according to an ordered lattice periodicity, wherein heights ofthe columnar structural units are all the same, and fall within an orderof detected wavelength; diameters of the columnar structural units fallwithin an order of subwavelength; a periodicity of an array composed ofthe columnar structural units is less than 10 microns, and a diameter ofthe array composed of the columnar structural units is equal to theaperture of the large-aperture metalens.
 4. The large-aperture infraredmetalens camera according to claim 3, wherein a material of the columnarstructural units comprises silicon or germanium; a material of themicrostructure array film coating comprises zinc sulfide or germanium; amaterial of the substrate comprises intrinsic double-thrown silicon,zinc sulfide or barium fluoride; and a material of the substrate filmcoating comprises zinc sulfide or germanium.
 5. The large-apertureinfrared metalens camera according to claim 4, wherein the aperture andan F-number of the large-aperture metalens are determined according tofollowing formulae:$C = \frac{1}{( {\frac{L}{D \times F} - 1} ) \times P} > C_{d}$$SNR = K \times \frac{P}{4F^{2}} > SNR_{d}$ wherein C is a pixel densityof the target object in a detection image, L is a distance between thetarget object and the large-aperture infrared metalens camera, D is theaperture of the large-aperture metalens, F is the F-number of thelarge-aperture metalens, P is a pixel spacing of the infrared focalplane array detector, C _(d) is a required minimum pixel density of thetarget object in the detection image; SNR is a signal-to-noise ratio ofdetection of the large-aperture infrared metalens camera, K is aparameter related to a radiation degree of the target object, detectionsurroundings, a lens transmittance, and a detector responsivity, andSNR_(d) is a required minimum signal-to-noise ratio of thelarge-aperture infrared metalens camera.
 6. The large-aperture infraredmetalens camera according to claim 5, wherein the metasurfacemicrostructure array is designed by following methods: optimallydesigning and obtaining a surface phase distribution of thelarge-aperture metalens by using a diffraction design algorithm or a raytracing algorithm according to the aperture and the F-number of thelarge-aperture metalens; obtaining a relationship between a phase and atransmittance of the columnar structural units and a size of thecolumnar structural units; determining the size of each of the columnarstructural units at each position in the metasurface microstructurearray according to the surface phase distribution of the large-aperturemetalens and the relationship between the phase and the transmittance ofthe columnar structural units and the size of the columnar structuralunits, and using the diffraction design algorithm or the ray tracingalgorithm again for feedback optimization; wherein an infraredtransmittance of the columnar structural units is greater than arequired value of the infrared transmittance.
 7. The large-apertureinfrared metalens camera according to claim 6, wherein the metasurfacemicrostructure array is manufactured by using a semiconductortechnology, comprising but not limited to stepping photolithography,step-scanning photolithography, nanoimprinting, laser direct writing,metal lift-off or ICP etching; the microstructure array film coating andthe substrate film coating are manufactured by an optical coatingprocess, comprising but not limited to electron beam evaporationcoating; the substrate is processed through optical polishing.
 8. Thelarge-aperture infrared metalens camera according to claim 1, whereinthe infrared focal plane array detector comprises a detector window andan infrared focal plane array, and the detector window and the infraredfocal plane array are sequentially arranged along an incident-lightdirection; the detector window is configured to filter out the straylight and the light outside the wavelength band of detection; theinfrared focal plane array is configured to detect and image theconverged light.
 9. The large-aperture infrared metalens cameraaccording to claim 1, wherein the metalens mechanical assembly comprisesa buffer structure, the buffer structure is provided with a groovematching an edge of the large-aperture metalens, the groove is providedwith a mechanical damping member therein, and the mechanical dampingmember is configured to fix and protect the large-aperture metalensagainst shocks; a material of the mechanical damping member comprisesbut not limited to a rubber, a composite material or a high dampingalloy.
 10. The large-aperture infrared metalens camera according toclaim 1, wherein contact surfaces where the housing is in contact withthe metalens mechanical assembly and the infrared focal plane arraydetector are respectively provided with sealing gaskets; the housing isprovided with a thermal insulation coating, and a material of thethermal insulation coating comprises but not limited to metal oxidemicro-powder or non-metallic hollow micro-spheres.